The present invention relates generally to a hermetic compressor and, more particularly, to such a compressor having a pressure relief system for reducing internal pressure during a high pressure condition.
Hermetic compressors comprise a hermetically sealed housing having a compressor mechanism mounted therein. The compressor mechanism may include a crankcase or a cylinder block defining a plurality of compression chambers in which gaseous refrigerant is compressed and subsequently discharged into a common discharge cavity.
Normally, the refrigerant returning to the compressor is in the gaseous state. At times, system conditions allow liquid refrigerant to return to the compressor via the suction line as a liquid or liquid/gas mixture. At these times, the compressor encounters a condition wherein the refrigerant is a liquid entering the cylinder. The travel of liquid refrigerant through a compressor is called "slugging".
A slugging condition is harmful to a compressor because many compressor parts, such as discharge valves, are not designed to cope with the high pressure condition caused by compressing liquid and the pressure entered by the liquid refrigerant as it moves through the restrictions of the system. A system that could prevent or reduce the affect that the liquid slugging has on the compressor components such as the discharge valve would lengthen compressor life.
Additionally, slugging causes a pressure backup from the discharge muffler and applies a very high back pressure to the piston. This pressure transfers back through to the connecting rods and/or yokes of the assembly thereby causing damage.
Some prior art compressor systems include a pressure relief valve to control the pressure within the cylinder head of the compressor. It has been found that these prior art systems are often not adequate, in that they do not act quickly enough to prevent damage to the valves and other compressor parts. Suction and discharge valves require certain minimum, discrete amounts of time to open, and the impact of a liquid slug can damage the valves before they can open. Further, most valves are not large enough to handle the volume or size of the liquid moving through the system. Even if the valve is open during contact with the liquid refrigerant, the valve port may not be large enough to adequately handle the flow of refrigerant and its impact pressure.
An example of a prior art compressor that tries to satisfy pressure relief in a compressor experiencing an overpressure condition is found in U.S. Pat. No. 3,125,285. In this patent, the cylinder is topped with a valve disc, beneath the cylinder head. Normally, a coil spring biases the disc into seated engagement with the cylinder wall. During the surge of the overpressure condition, the valve disc will be elevated against the biasing force of the spring into contact with a stop ring, thereby opening a passageway between a discharge chamber and a suction pressure chamber. As stated before, the valve disk takes time to open and sometimes is not large enough to adequately handle the amount of liquid present.
An additional problem with prior art pressure relief systems, including the one above, is that of creating refrigerant leaks from high pressure areas to low pressure areas. Possible leaks are caused by high to low pressure boundaries on metal to metal seals. Refrigerant leaking between a high pressure area to a low pressure area during compressor operation causes the compressor and refrigerant system to operate less efficiently.
The present invention is directed to overcoming the aforementioned problems associated with hermetic compressors wherein it is desired to control overpressure conditions and reduce damage to the compressor while not reducing efficiency.